I wrote about this in some detail in The Sorcerer's Soul. As GM, I have observed the master of a possessor to permit it into his or her body on several occasions. Surprisingly, all details about possession, of whom and what happens, turn out to work very well as long as the rules are followed in detail, and I strongly recommend the supplement toward that end.

Any possessor can try to possess anyone. However, do note the importance of Hop. A possessor without Hop is much more constrained.

Regarding "permission" for a possessor to inhabit its master, there isn't any such thing. The demon does what you, as demon-player (GM), say it does. If you want it to try, for whatever reason of the moment, then it does.

Nothing about being possessed removes a player-character from play, nor does it bring you, as GM, into control of that character - just his or her body. He or she can still try to break free either momentarily or for good, can still command the demon to do things, and can still "be in the loop."

Your point about Need is a good one. A possessor in the body of its binder must "loose the reins" in order to get its Need fulfilled, and that gives the binder a good bargaining chip about getting it out if that's what he or she wants.

Suffice to say that by choosing a possessor as a starting demon, the player has already entered into an agreement that the risk of having his body co-opted is acceptable. That is not to say that all possessors necessarily want to inhabit their masters, or that such an act will become part of play at all, but it is a possibility inherent in the Type.

Briefly, most of this is handled by Will vs. Will rolls, modified by the Binding bonus if the master is the host.

Yes, the possessor can jump in the back seat, and any disagreements about any seat-shifting would be handled by such a roll. Same goes for the host trying to get control for a single action, for a moment. And so on, up to and including kicking the demon out, which is effectively a straightforward command.

And yes! A willing host means no roll. Good point. I'm pretty sure I don't mention that in the book.

OK, almost crystal clear. What is the mechanical difference between gaining momentary control vs. kicking the bastard out? Any victories and I get momentary control, but I need a certain number of (possibly cumulative) victories to kick the demon out? Greater than the demon's Power?

I would say to give a player only attempting momentary control bonus dice to their roll. For example: Jeffery binds a demon far more powerful than would normally be bound, to top this he often mistreats the demon not realizing that he does not have as tight a bind as he thinks. The demon takes over Jeffery and starts trolling around in his body indulging in its drinking habits (need), and sexual (desires). Jeffery makes a couple attempts to boot the demon but has no success so he takes a new strategy. Jeffery starts to each time the demon begins to indulge its drinking need take over for just a couple seconds or minutes and spills the demons drinks destroying any bottles near him. Even though it will take him a while each week Jeffery can starve the demon of its need he can gain ground on the demon. Mind you the demon will most likely do everything in its power to destroy Jeffery's social life in the mean time, so when he gets control he might have lost his job, friends, loved ones, house.

Actually, re-reading Ron's post for the third or fourth time, I think he's saying that "Take a back seat" and "Get thee hence!" are totally equivalent commands and would both be simple Will v Will rolls, modified as usual. In both cases the Sorcerer is just giving the demon a command, handled like any other command.

John's right. The roll to boot the demon out is the same as the one to take over briefly.

One may ask, why is that? Why don't I "get" the whole enchilada when I make the roll?

The answer is, you would, if that's what you wanted and said you were trying to do. If you didn't want it (i.e. you instead wanted to stay possessed, just not for this moment), then why ask why you don't get it?

A character in a game a while ago had a Stamina of 1, and was furthermore hampered by the descriptor of almost total paralysis. She commanded a Possessor demon with Hop who was hands-down the most terrifying demon I've ever seen in playing Sorcerer. At one point, she had the demon possess her in order to get around and do some stuff. It was very risky - the demon had to keep its Power and other ability use tamped way down to keep from risking her life - but it was absolutely necessary for what she wanted to do.

So when she wanted to 'take the reins' once in a while, that's all she wanted at the moment. Getting the demon out of her body wasn't a concern until later. And as it happened, when that did happen, there was no conflict, as the demon was very happy with their Binding circumstances, knew that its master was safer (physically) when not possessed, and knew that it could cut loose with its abilities much more successfully in other hosts.